On the Stability of Cu5 Catalysts in Air Using Multi-Reference Perturbation Theory

Abstract

An ab initio study of the interaction of O2 the most abundant radical and oxidant species in the atmosphere, with the Cu5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for using multi-reference perturbation theory, allowing to investigate the experimentally confirmed resistivity of Cu5 clusters towards oxidation. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multi-reference approach, the transition barrier for O2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu5 arranged in a bipyramidal structure, the O-O bond cleavage is confirmed as the rate-determining step. However, for planar Cu5, the high energy barrier for O2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case.